A touch sensor is an input device for sensing a point that has been touched with a finger or a pen. Examples of known location sensing techniques include electrostatic capacitive coupling type, a resistive film type, an infrared ray type, an ultrasonic type, and electromagnetic induction/coupling type. Among other things, touch sensors of the resistive film type or electrostatic capacitive coupling type are used extensively.
Hereinafter, a touch sensor of the resistive film type will be described. As shown in FIG. 22, a touch sensor of an analog resistive film type includes two transparent resistive films 12 and 14 facing each other with an air layer 13 interposed between them, a PET film 11 provided on the opposite side of the transparent resistive film 12 so as to not face the air layer 13, and a glass film 15 provided on the opposite side of the transparent resistive film 14 so as to not face the air layer 13. Of the two transparent resistive films 12 and 14, one transparent resistive film 12 is provided with a pair of conductive portions 16 that are spaced apart from each other in a Y-axis direction. The other transparent resistive film 14 is provided with a pair of conductive portions 17 that are spaced apart from each other in an X-axis direction. Alternatively, the transparent resistive film 12 may be provided with the conductive portions 17 and the transparent resistive film 14 may be provided with the conductive portions 16 instead.
In a touch sensor of this resistive film type, when the operating plane thereof is touched with a finger, for example, the transparent resistive films 12 and 14 contact, and become electrically continuous, with each other at that point of contact (i.e., press point), thereby obtaining the coordinates of the point of contact.
A voltage is applied between the pair of conductive portions (e.g., conductive portions 16) of one of the two transparent resistive films 12 and 14 (e.g., transparent resistive film 12). In this case, when the resistive films 12 and 14 contact with each other, the resistive films 12 and 14 become electrically continuous with each other and current flows through the transparent resistive film 14. Accordingly, by detecting the voltage of the transparent resistive film 14, the Y coordinate of the point of contact can be detected based on that voltage value.
Hereinafter, a specific example will be described. For example, a voltage gradient is created in the transparent resistive film 12 by applying a voltage of 0 V to one of the two conductive portions 16 of the transparent resistive film 12 and a voltage of 5 V to the other conductive portion 16, respectively. Meanwhile, no voltage is applied to the other transparent resistive film 14. In this case, if contact is made at the center of the transparent resistive film 14, for example, a voltage of 2.5 V, which is half of 5 V, is detected from the transparent resistive film 14. Alternatively, if contact is made at a point that is located close to the conductive portion to which 5 V has been applied, a voltage close to 5 V is detected from the transparent resistive film 14. As another alternative, if contact is made at a point that is located close to the conductive portion to which 0 V (i.e., no voltage) has been applied, a voltage close to 0 V is detected from the transparent resistive film 14. In this manner, the Y coordinate of the point of contact can be detected based on the voltage that has been detected from the transparent resistive film 14.
In detecting the X coordinate of the point of contact as in the Y coordinate thereof, a potential difference is created between the pair of conductive portions 17 of the transparent resistive film 14, no voltage is applied to the transparent resistive film 12, and the voltage at the point of contact is detected by way of the transparent resistive film 12. By alternately detecting the X and Y coordinates in this manner, the press point can be located.
In the touch sensor of this analog resistive film type, the air layer 13 is provided between the two transparent resistive films 12 and 14 to make the two transparent resistive films 12 and 14 electrically continuous with each other ay the press point. However, if the air layer 13 is present, then reflection will occur at the interface between the transparent resistive film 12, 14 and the air layer 13 due to the difference in refractive index between the transparent resistive film 12, 14 and the air layer 13. Accordingly, an image display device equipped with a touch sensor of such an analog resistive film type has a darkened display. A touch sensor of such an analog resistive film type is disclosed in Japanese Utility Model Laid-Open Publication No. 5-4256.
On the other hand, a touch sensor of an analog electrostatic capacitive coupling type typically includes a single location sensing transparent conductive film. As disclosed in Japanese National-Phase Publication 56-500230, in a touch sensor of the analog electrostatic capacitive coupling type, an alternating current voltage with the same phase and same potential is applied from each of the electrodes at the four corners of the location sensing transparent conductive film, and a substantially uniform electric field is applied to the entire location sensing transparent conductive film.
When a point of contact is given at some location on the location sensing transparent conductive film, currents flow from the four corners of the location sensing transparent conductive film. And by measuring the amounts of currents at these four corners, the X and Y coordinates of the point of contact can be detected.
Comparing the touch sensor of the resistive film type with the touch sensor of the electrostatic capacitive coupling type, the touch sensor of the latter type has a higher transmittance than the touch sensor of the former type because the touch sensor of the former type needs an air layer but the touch sensor of the latter type doesn't. In addition, the touch sensor of the electrostatic capacitive coupling type ensures better impact resistance and dustproofness than the touch sensor of the resistive film type, and therefore, is able to operate properly for a longer time even if somewhat soiled.
Even a touch sensor of the electrostatic capacitive coupling type, however, is supposed to be combined with a display panel. Accordingly, a touch sensor that can further check the decrease in transmittance of the display panel is now in high demand. Furthermore, a display panel including such a touch sensor as an integral part thereof needs to further reduce its size and weight.
Also, in the touch sensor of the analog electrostatic capacitive coupling type as disclosed in Japanese National Phase Publication No. 56-500230, a complicated electrically conductive segment pattern 19 such as that shown in FIG. 23 needs to be provided around the periphery of the location sensing transparent conductive film 18 to locate the point of contact accurately enough. If the conductive pattern gets complicated, however, then the non-effective range where no location sensing can be carried out expands. For that reason, a touch sensor of an electrostatic capacitive coupling type including simpler circuits is now in high demand.